I'm not sure about Houston, but in L.A and San Diego there are holding areas at the airport where the black cars hang out. I would find out if your city has a similar holding area at the airport, and just go down there and start talking to drivers. Another idea is to be a pax for an SUV, and ask the driver about where they SUVs hang out at at the airport. There usually is an area of some kind, if only a nearby Starbucks parking lot, etc. Also, go on the Black drivers forum or on the Houston forum on UP, and ask.